The invention relates to an alloy containing a fraction of particles and to a process for producing objects from such an alloy, in particular with an addition of particles in an aluminum alloy, so as to increase the wear resistance.
Aluminum alloys with a fraction of particles, in particular in the form of primary silicon crystals, which increase the wear resistance are known, for example, as hypereutectic aluminum-silicon casting alloys. These alloys can be used to cast, for example, entire engine blocks or alternatively cylinder liners. The primary silicon crystals precipitate out during cooling. The wear resistance of, for example, the running surface is provided by the precipitated, harder primary silicon crystals, which are exposed at the surface by special treatment processes, in particular etching processes. A drawback of these hypereutectic aluminum-silicon casting alloys is that the primary silicon crystals are in the form of sharp-edged, sometimes acicular crystals and in different sizes and distributions depending on the solidification rate, so that machining requires special tools in order to keep the wear caused by the hard primary silicon crystals within acceptable limits.
Producing entire cylinder blocks from a hypereutectic aluminum-silicon alloy is expensive, since this material requires increased outlay during casting and, as mentioned above, high machining outlay owing to the precipitated primary silicon crystals.
In order to avoid these difficulties in machining large castings, it is also already known, for example, to fit, in particular by casting, cylinder linings made from a hypereutectic aluminum-silicon alloy in a cylinder block produced from a conventional, readily castable aluminum alloy.
While the wear-resistant, hypereutectic aluminum-silicon casting alloys for cylinder blocks normally have a silicon content of about 17% by weight, the separately produced cylinder liners may have silicon contents of from 20 to 30% by weight, in which case, for example, initially a billet is produced by spray compacting of a hypereutectic aluminum-silicon alloy or by powder metallurgy from a powder of such a hypereutectic aluminum-silicon alloy, and then the liner is produced from this billet by hot extrusion. These hypereutectic aluminum-silicon alloys, and the processes for producing cylinder liners, are described in German Patent 43 28 619 and German laid-open specification 44 38 550.
If only the cylinder liners are produced from a wear-resistant, hypereutectic aluminum-silicon alloy, a drawback of both the spray compacting of a hypereutectic aluminum-silicon alloy and the powder metallurgy process is that billets which consist entirely of a hypereutectic aluminum-silicon content of up to 30% by weight, the intermetallic phases and the primary silicon crystals which bring about the wear resistance crystallize out of the molten hypereutectic material during cooling and consequently have the sharp-edged and acicular shapes which are typical of primary silicon crystals. In order to reduce the wear on pistons sliding inside the cylinders of such engine blocks caused by these primary crystals and intermetallic phases, it is proposed, according to German Patent Application 44 38 550, for the primary silicon crystals and particles from intermetallic phases to be exposed by means of precision machining and to change the edges of the exposed surfaces of the primary crystals or particles into a convex or rounded shape in the base alloying material.